1. Field of the Invention
The present invention relates generally to permanent magnets and, more particularly, to high intensity permanent magnets having gradient fields and to methods of making such magnets.
2. Related Art
Permanent magnets that are capable of producing high intensity magnetic fields and that have a compact structure are known and are used, e.g., in miniaturized electrical components including disk drives for laptop and palmtop computers. In particular, permanent magnet materials that are highly remanent and coercive, such as those of the rare earth type, are produced to make compact flux sources of extraordinary strength. Examples of high-intensity, compact permanent magnets, which may employ these materials, may be found in the following patents. U.S. Pat. No. 4,837,542, to Leupold, entitled “Hollow Substantially Hemispherical Permanent Magnet High Field Flux Source for Producing a Uniform High Field”; U.S. Pat. No. 4,839,059 to Leupold, entitled “Clad Magic Ring Wigglers”; U.S. Pat. No. 5,103,200 to Leupold, entitled “High-Field Permanent Magnet Flux Source”; U.S. Pat. No. 5,216,401 to Leupold, entitled “Magnetic Field Sources Having Non-Distorting Access Ports”; U.S. Pat. No. 5,382,936 to Leupold et al., entitled “Field Augmented Permanent Magnet Structures”; U.S. Pat. No. 5,426,338 to Leupold, entitled “High-Power Electrical Machinery with Toroidal Permanent Magnets”; U.S. Pat. No. 5,434,462 to Leupold et al., entitled “High-Power Electrical Machinery”; and U.S. Pat. No. 5,523,731 to Leupold, entitled “Simplified Method of Making Light Weight Magnetic Field Sources Having Distortion-Free Access Ports. The entire contents of each of the foregoing patents is hereby incorporated herein by reference to the extent necessary to make and practice the present invention.
The basic configuration from which the magnetic arrangements described above are derived may be referred to as a Halboch Structure or a magic cylinder or ring. The magic ring is a permanent magnet which is magnetized in accordance with the configuration shown in FIG. 1. The orientation of magnetization at any point (P) is at an angle (γ) from a vertical axis (z) and is equal to twice the polar coordinate of P,(θ) or according to equation (1) as follows:γ=(2)(θ)  (1) where:                (θ) is a polar angle between the x and z axes that may vary from θ=0° to θ=±0° as shown.Such a magnetization pattern produces in an internal cavity (c) a uniform magnetic field (represented by arrow h). Ideally the change in direction of magnetization should be continuous but this is not technically feasible. Instead an approximation to the ideal magic ring structure can be formed by a method as described in U.S. Pat. No. 5,523,731, previously incorporated herein by reference. The method may comprise uniformly magnetizing a cylindrical shell along a plane defined by radii of the shell and cutting the shell into thin washer shaped pieces. Each of the washer-shaped pieces may then be cut in a radial manner to form truncated pie shaped pieces that may be then reversed 180° (turned upside down) and transposed to proper locations along the circumference of the ring to form a thin magic ring slice having the magnetization approximately as shown in FIG. 1. The formation of a magic cylinder may be accomplished by stacking the magic rings together in an elongated fashion. It will be recognized that the ideal structure can be approached as closely as desired by increasing the number of truncated pie shaped pieces per magic ring.        
It may be desired in particular applications employing magic rings or cylinders that access ports of various sizes, shapes and locations extend through the shell and communicate with the internal cavity (c). However, removal of magnetic material to provide an access port to the interior through the magnetic shell will distort the interior field especially in the vicinity of the port. To overcome this drawback, a further method is proposed, as also described in U.S. Pat. No. 5,523,731, wherein some of the thin washer-shaped pieces that are sliced from the uniformly magnetized cylinder are interleaved with the magic ring slices to form a cylinder that allows for non-distorting access ports. In such a case, removal of magnetic material results in a non-distorting access port, since a uniformly magnetized ring produces no field in its interior cavity and superposition of such a magnetization pattern upon that of ae magic ring would result in no change in the field located in the cavity of the magic ring.
It is also sometimes desired to produce permanent magnets having a shell and a cavity wherethrough a high intensity magnetic field extends which is tapered, or has a gradient. For example, electron-beam tubes often require gradient fields, which typically vary along a beam axis, for use in focusing and guiding the beam. Microwave and millimeter wave sources require an axial field variation of a longitudinal magnetic field in order to produce a crisp waveform and storage rings and particle accelerators may require transverse magnetic fields including field tapering in the direction of a longitudinal axis of a beam in order to compensate for changes in a velocity of the beam. In spectroscopic analysis, magnetic fields with a linear taper in the direction of the field are often used to produce a spectral distribution of absorbed or emitted electromagnetic energy.
U.S. Pat. No. 5,216,400 to Leupold (below referred to as the “400 Patent”), the entire contents of which is hereby incorporated herein by reference to the extent necessary to make and practice the present invention, describes a permanent magnet having a cavity and producing a magnetic field that varies in intensity and in the direction of its orientation to produce a tapered or gradient magnetic field within a cavity thereof. As generally described therein, to provide a linear taper along the z axis of a magnetic ring or cylinder where the south pole is at z=0 at an inner edge of a cavity, the remanence is tapered along the polar angle θ according to equation (2) as follows:Br(θ)=mθ+BrMin(0°)  (2) where:                Br(θ) is the desired magnet remanence at θ;        BrMin is the minimum remanence appropriate to produce a field H(Min) at the low end of the taper located in the cavity of the magnetic ring or cylinder and H(Max) being the field at the high end of the taper; and        where m is found according to equation (3) as follows:m=[BrMax(90°)−BrMin(0°)]/π  (3) where:        BrMax is the maximum remanence at z=dl at the north pole of the cavity and di is the diameter of the cavity.        
The permanent magnets described in the above patents and documents have numerous applications and advantages, however, it is desired to provide a permanent magnet including a cavity having both a gradient magnetic field and a distortion free access port.